DENVER — The Tri-County Health Department, which serves Adams, Arapahoe and Douglas counties, announced July 30 it is recommending that all people wear masks in indoor settings, regardless of vaccination status.

The change reflects updated CDC guidance, which recommends mask-wearing for anybody who is “in public indoor settings in areas of substantial or high transmission.”

Almost every Colorado county is experiencing substantial or high transmission.

[Related:Do You Need To Wear A Mask Indoors Where You Live? Check This Map]

“One thing that we have learned over and over again during the pandemic is to expect the unexpected,” said Dr. John M. Douglas, Tri-County Health Department’s executive director, in a news release. “Although we think a resumption of wearing masks in schools and public indoor settings can be a useful measure to stem increases in transmission, it’s quite clear that getting vaccinated as soon as possible is the best way to prevent the spread of COVID-19.”

So far, Denver County has not made a similar announcement.

Tri-County’s updated guidance comes as Colorado and the U.S. as a whole battle a surge in COVID-19 cases caused mostly by the Delta variant, a highly transmissible version of COVID-19. The Delta variant—which new evidence shows is more contagious than Ebola and just as contagious as chickenpox—is now believed to make up 95% of new cases in Colorado. It was first detected in Colorado in Mesa County.

To date, there have been more than 570,000 confirmed cases of COVID-19 in Colorado. Nearly 33,000 people have been hospitalized, and over 7,200 people have died from the virus. The latest COVID-19 data, including daily case counts, are available here.

Over a quarter of Colorado's population lives in either Adams, Arapahoe or Douglas counties. At 71.8%, Douglas County is the only one of the three counties that has vaccinated over 70% of its eligible population. But the others aren’t far behind. 65.8% of eligible Adams County residents are vaccinated. In Arapahoe County, that number is at 69.6%. Get the latest county-level vaccination data here.

“We are in a race against time to get more people vaccinated before Delta spreads even further or new even more contagious variants emerge,” Douglas said.

Compared to other states, Colorado is doing well in vaccinating eligible residents. More than 3.35 million people have received at least one dose. Over 70% of adults have received at least one dose and as of July 30, 69.5% of the eligible population (Coloradans ages 1 and up) have received the first shot of their vaccine.

“Colorado has been administering the vaccine for over eight months and it has proven to be a safe and effective defense against this destructive virus,” Governor Jared Polis said in a July 29 news release. “This is now becoming a pandemic of the unvaccinated and we have the tool to end it. Now more than ever it could not be more clear that you are either on the side of spreading this virus or you are on the side of helping the state get back to the Colorado we know and love.”

Despite the increase in vaccinations, there have been more “breakthrough” cases. Those happen when a fully vaccinated person still tests positive for COVID-19. As Laura Santhanam wrote for PBS NewsHour, public health experts say breakthrough cases “are not a sign that vaccines are failing. Instead, they are a warning of how vaccine holdouts can endanger even their inoculated neighbors.”

Dr. Alex Huffman, an aerosol scientist and professor at the University of Denver, told NewsHour that breakthrough cases are to be expected with any vaccine, but that these cases also speak to just how contagious the Delta variant is. Huffman said that when people are infected with the Delta variant, they produce a thousand times greater viral load than seen from the original COVID-19 strain.

Moreover, recent data shows that fully vaccinated people can easily carry and transmit the virus, though they are far less likely to become seriously ill, require hospitalization, or die. 97% of people hospitalized with COVID-19 have not been vaccinated, NewsHour reports.

To find out where to get a vaccine in Colorado, click here.

Kyle Cooke is the Digital Media Manager for Rocky Mountain PBS. You can reach him at kylecooke@rmpbs.org.